1. Technical Field
The present invention relates to a battery management system which includes a secondary battery module, and a driving method thereof, and more specifically, the invention relates to a method of controlling a secondary battery module for stably managing a battery according to a cut-off voltage of the secondary battery module.
2. Related Art
Recently, high power secondary batteries using non-aqueous electrolytes having a high energy density have been developed, and a large capacity secondary battery is formed by coupling a plurality of high output secondary batteries in series so that the secondary battery can operate a motor of an apparatus requiring high power (e.g., an electric vehicle).
As described, one large capacity secondary battery (hereinafter referred to, for convenience of description, as a battery module) includes a plurality of secondary batteries (hereinafter referred to as a unit battery) coupled in series.
In the above battery module, more specifically, in a secondary battery module for a hybrid electric vehicle (HEV), several to tens of unit batteries are alternately recharged and discharged. Therefore, it is required that the charge/discharge operation of the battery module be controlled so as to maintain the battery module in an appropriate operational mode.
For example, when the battery module is charged and used, the respective unit batteries forming the battery module are repeatedly charged and discharged, during which energy levels of the respective unit batteries become different from each other. When a plurality of unit batteries coupled in series are recharged after they are once discharged (i.e., used) to different energy levels, the energy levels of the recharged unit batteries also become different from each other. In this case, when the charge and discharge operations are repeatedly performed, some of the unit batteries forming a group are over-discharged so that output potentials thereof becomes less than 0V. When a user continuously uses the over-discharged unit batteries and discharges them, a battery reversal occurs so that potentials of the unit batteries are reversed.
As described, when unit batteries having different respective energy levels are coupled in series and charged, the unit batteries having higher energy levels transmit a charge completion signal to a charger before the unit batteries having lower energy levels are charged, and the charger prematurely finishes the charge operation. In addition, when the battery module includes the over-discharged unit battery, the unit batteries other than the over-discharged unit battery are over-charged before the over-discharged unit battery is fully charged. That is, incomplete charge and over-discharge operations are repeatedly performed or the battery reversal repeatedly occurs in some of the plurality of unit batteries, and complete charge or over-charge and incomplete discharge operations are repeatedly performed on the others of the plurality of unit batteries, and therefore the unit batteries are damaged.
Therefore, to reduce damage of the unit batteries, the conventional secondary battery module includes a battery management system (BMS) for managing states of the respective unit batteries and a relay for preventing current transmission when the battery module is out of order.
The BMS detects voltages of the respective unit batteries in the battery module, controls the relay to perform a cut-off operation when the detected voltage of the unit battery is higher or lower than a cut-off voltage which is a reference value, cuts off the current of the battery module, and recovers the unit battery.
The cut-off voltage is conventionally set to a range that is lower than 2.8V or higher than 4.3V. In this case, there is a problem in that the battery module is not sufficiently recovered since the relay is turned on when the voltages of the respective unit batteries are higher than 2.8V or lower than 4.3V while the voltages of the respective unit batteries are recovered to within the range of the cut-off voltage.
The above information is disclosed herein only for enhancement of an understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art in this country.